Gordon Brown today announced the second review in two years into whether cannabis should be reclassified, in response to concerns that its current status does not reflect the drug's dangers.

Mr Brown announced the review, which will look at whether cannabis should be reclassified as class B again - rather than its present class C - at prime minister's questions.

It was his second major policy announcement at PMQs in seven days, after last week announcing a U-turn on supercasinos.

While the Guardian revealed yesterday that a statement on crime reduction would be among a flurry of policy statements before parliament goes into recess next week, the announcement of another look at marijuana came as a surprise.

Mr Brown said that the home secretary, Jacqui Smith, would publish a consultation document next week to review drugs strategy.

"She will be asking the public to comment on new ways in which we can improve drugs education in the country, give support to people undergoing treatment ... and give support for communities who want to chase out drug dealers from their communities.

"As part of the consultation - and the cabinet discussed this yesterday - the home secretary will also consult on whether it is now right that cannabis should be moved from class C to class B."

Mr Brown was responding to Labour's Martin Salter, who wanted to see more prescriptions of cannabis-extract Savitex to people with multiple scelorsis.

Cannabis was class B before being downgraded to class C after a change in the law in 2004 under the then-home secretary, David Blunkett.

As a class B drug it would carry much more severe penalties for possession and imprisonment.

The prime minister's spokesman later said: "Clearly these issues do need to be kept under review. There have been reports of stronger strengths of cannabis on the market."

Iain Duncan Smith, the former Tory leader, called for cannabis to be reclassified in his Breakthrough Britain report published last week.

There have been particular concerns about skunk, a very potent form of cannabis which has been linked to mental health disorders.

In 2005, 10,000 11- to 17-year-olds were treated for cannabis use - 10 times the number a decade ago.

Plants are increasingly cultivated to include high levels of the active ingredient of cannabis, THC, delta-9-tetrahydrocannabinol, which encourages addiction and can cause a range of symptoms, from short-term memory loss, anxiety and panic attacks to triggering schizophrenia.

Charles Clarke, the former home secretary, launched a review last year but concluded that there was no need to change the classification despite the changing medical advice.

Danny Kushlik, director of drug policy foundation Transform, said cannabis would be better controlled and moderated if its use was not prohibited, and accused Gordon Brown of playing to the gallery.

"It's got nothing to do with science and everything to do with a Dutch auction over who can be the toughest on drugs, sadly," he said.

"There's no new evidence. As Gordon Brown's first Commons statement on drug policy, it's shameful."

Paul Corry, director of public affairs at mental health charity Rethink, which has warned of the dangers of cannabis, said the government was exerting its energies in the wrong place.

"Any movement to reclassify cannabis would be a waste of time and resource," he said.

"All the evidence shows the risk of mental illness and that the criminal justice system is not able to deal with that. A long-term and sustained public health campaign is needed.

"What's not good is criminalising a group of people at risk of developing mental illness."